In the field of quantum information such as quantum communication and quantum cryptography, there is a need to reliably produce single photons. One photon source which has been previously suggested for producing single photons is based on quantum dots. In a quantum dot, an exciton is formed when there is a bound state between a small number of electrons in the conduction band and holes in the valence band, radiative decay occurring when one hole and one electron recombine resulting in the emission of a photon. Due to the Pauli Exclusion Principle each transition cannot give rise to the emission of two photons at the same time. These photons can be used as “flying quantum bits” to carry quantum information in an application, where the information is encoded on the polarisation of the photon, its energy or the spatial mode it travels in.
A desirable property of such photons is that they are coherent, that is with a narrow spectrum which is invariant in time, all photons being identical. The “natural linewidth” of the transition, /τr, is determined by its radiative lifetime (τr), if this is the case the transition is said to be homogeneously broadened. This property allows the quantum “bits” of information to display two-photon interference, which is essential for exchanging information between them, and thus building logic gates which can manipulate the quantum information. In the field of quantum cryptography, two photon interference is an essential component of quantum repeater schemes needed to increase the distance over which information may be sent. In addition, coherent photons are preferred for some quantum-optical metrology applications as their narrow spectrum increases the resolution of the system.
Emission from solid state light sources is often inhomogeneously broadened, with a linewidth greater than the “natural linewidth” due to dephasing and spectral jitter. The visibility of two photon interference for a pulsed inhomogeneously broadened light source is approximated to the ratio τc/2τr where τc is the “coherence time” of the spectrum (which is equal to h/π*dE where dE is the width of transition) and τr is the radiative lifetime. Fluctuations in the central energy of the transitions between photon emission events will further reduce the visibility. With non-resonant excitation it is challenging to obtain a high visibility of interference in solid state systems, due to the excitation of excess carriers in the structure. However, direct excitation of the transition at the same energy as the emitted photon can eliminate this problem and lead to photons being emitted with very narrow linewidth.